1. Field of the Invention
The present invention relates generally to a recording and reproducing apparatus, and particularly to recording and reproducing apparatus for video signal on a recording tape by using rotary heads, and more particularly to a VTR for recording and reproducing video signal on and from a magnetic recording tape by using rotary recording and reproducing heads.
2. Description of the Prior Art
In the conventional video tape recorder wherein plural rotary magnetic heads are disposed with uniform angular pitch on a rotary head cylinder and a recording tape, e.g., magnetic tape is wound around the rotary head cylinder in helical manner with a predetermined angle. Therein, the plural rotary heads are switched for operation in turn to sequentially record or reproduce the video signal for one field on a recording track disposed oblique with respect to the longitudinal direction of the recording tape. In such recording and reproducing apparatus of helical scanning type, an improvement has been proposed as shown in FIG. 6 for decreasing diameter of the rotary head cylinder (Japanese Unexamined Published Patent Application Sho No. 45-20403).
That is, as shown in FIG. 6, the recording and reproducing apparatus of the prior art comprises four magnetic heads 21, 22, 23 and 24 which are disposed on the cylindrical surface of a rotary head cylinder 20 with uniform 90.degree. angular pitches with each other. And among them, each pair of two magnetic heads are disposed with 180.degree. angle pitch each other. And, a pair of the magnetic heads 21 and 23 have the same azimuth angle each other; and the other pair of the magnetic heads, namely, the magnetic heads 22 and 24 have another same azimuth angle which is different from that of the former one. A magnetic recording tape 4, face of which is schematically drawn in an upper part of FIG. 6 show the recording track, is wound around the rotary head cylinder 20 for over 270.degree. angle of the cylindrical surface, by means of known tape driving means 104, 105. In the above-mentioned configuration, when the magnetic heads are switched in an order of the magnetic heads 21, 22, 23, 24, 21, 22 . . . , oblique tracks A, B, A', B', A, B . . . are obtainable on the magnetic recording tape 4 as shown in FIG. 6.
At this time, since there is a necessity that video signal for one field is recorded on one recording tracks A, B, A' or B', the rotary head cylinder 20 must be driven to rotate in synchronism with vertical synchronization signal of the video signal to be recorded. That is, the rotary head cylinder 20 must be rotated for 3/4 turn during one period of the vertical synchronization signal, keeping accurate phase synchronization with the vertical synchronization signal.
Accordingly, in the prior apparatus, output signal of a rotary phase detector 7 is divided to 1/3 by a 1/3 frequency divider 8 thereby producing a signal of 15 Hz which is the greatest common between 45 Hz of output of the rotary phase detector 7 of the rotary head cylinder 20 and 60 Hz (in case of NTSC) of vertical synchronization signal of television video signal to be recorded. And on the other hand, by dividing the vertical synchronization signal S.sub.v obtained from the television video signal to be recorded to 1/4 times by a 1/4 divider 9 thereby producing a second 15 Hz signal, and then by comparing phases of the above-mentioned two 15 Hz signals by a phase comparator 10, a cylinder motor 11 is servo-controlled so that both phases of the above-mentioned two signals coincide.
In the above-mentioned configuration of the prior art apparatus, the cylinder motor 11 is servo-controlled by using a signal having such a low frequency as 15 Hz. And therefore, the rotary head cylinder 20 is controlled of each phase by means of the 15 Hz signal which is issued for every 3 turns of the rotary head cylinder 20. Accordingly, the phase control in the prior art is not necessarily accurate enough.